Your hair follicles are rapidly dividing cells. That's why your hair thins.
Your intestinal lining regenerates every three to five days — some of the fastest-dividing tissue in the human body. That's why you feel nauseated, bloated, and gut-wrecked every Saturday morning.
Your oral mucosa regenerates continuously. That's why you develop mouth ulcers so severe you sometimes can't eat solid food.
Your liver cells regenerate. That's why your ALT and AST climb, why we monitor hepatic function every eight to twelve weeks, and why some of my patients have had to stop the drug entirely because their liver couldn't sustain the damage.
Your bone marrow cells divide rapidly. That's why methotrexate can cause myelosuppression — a reduction in the blood cells your body needs to fight infection, carry oxygen, and clot wounds.
I prescribe folic acid alongside methotrexate to partially offset the folate antagonism.
But I'll be honest with you: folic acid is a bandage on a structural problem. It reduces some side effects.
It doesn't change the fundamental mechanism.
The drug is still attacking every rapidly dividing cell in the body and relying on the collateral immune suppression to calm the joints.
I've known this pharmacologically for nineteen years.
But I didn't FEEL it until I was the one sitting on the bath edge every Friday night, watching my hair thin, losing every Saturday to a fog I couldn't think through, and wondering whether my liver would hold up for another decade.